As an electric motor that drives a dental handpiece including a tool, a brushless slotless motor is used, for the sake of the suppression of rotation unevenness and vibration (for example, Patent Literature 1 and Patent Literature 2).
A plurality of coils provided on a bobbin are disposed at the inner circumference of a cylindrical stator core of the motor.
The motor that is applied to the dental handpiece is provided with pathways through which water and air pass. The water and air introduced into the handpiece through those pathways are supplied from the position of the tool of the handpiece into the mouth cavity. In the case where the handpiece has a lighting function, the motor is provided also with a pathway through which an electric wire for lighting passes.
The dental motor requires a downsizing that allows an operator to easily perform operation in hand, in addition to a sufficient torque output (performance) for stably performing the cutting and polishing with the tool. In order to achieve a small high-performance motor, in the internal space of the case, it is desirable to reduce a region that does not contribute to the performance as much as possible, and secure a large region that can be effectively used on the magnetic circuit.
Here, as the kind of a coil (winding wire) of the slotless motor, there are a lap winding in which coils are disposed so as to lap over each other, and a non-lap winding in which coils are disposed so as not to lap over each other.
As an example of the lap winding, there is Patent Literature 1. In the lap winding, as shown in FIG. 11A, on the inside of an opening 100 of each of coils 101 to 103, a coil side 10s (coil straight) of another coil is disposed. Then, the coils 101 to 103 are lapped over each other, at coil ends 10e. 
FIG. 11B schematically shows a cross-section of a portion where the coil 101 and the coil 102 lap at the coil ends 10e. Hatched lines are given on a cross-section of the coil 102. The coil side 10s of the coil 102 is disposed on the inside of the opening 100 of the coil 101, while the coil end 10e of the coil 102 is lapped over the coil end 10e of the coil 101. In this way, as the whole of the coils 101 to 103 lapped at the coil ends 10e, the coil ends 10e are formed so as to be thicker than the coil side 10s, and protrude in radial directions relative to the coil side 10s. 
In the case of the lap winding, the pipes as the pathways for water and air and the electric wire for lighting are weaved into the interior of the coil ends 10e, and the coil is shaped. Thereby, it is possible to integrate the pipes and the electric wire with the coil. In such a complex lap winding structure, the coil tightly contacts with the pipes and the electric wire at the coil ends 10e. Therefore, an excessive stress is likely to be given on the coil, and the production cost is high. Further, because of the complex structure, the design change such as the increase in the space factor that is the proportion of conductors to the cross-section area of the coil is not easy.
Furthermore, in the lap winding, as shown in FIG. 11B, the coil ends 10e protrude in radial directions relative to the coil side 10s, and a stator core 105 whose diameter is set so as to be equivalent to the outer diameter of the coil side 10s cannot be provided to the coil ends 10e. Therefore, the length of the motor that can be effectively used is shortened by the length of the coil ends 10e, with the restriction of the length of the motor due to the requirement of downsizing. The whole circumference of the coil ends 10e is an ineffective region that does not contribute to the performance of the motor. Although the coil ends 10e protrude in radially outward directions in the above case, in the case where the coil ends 10e protrude in radially inward directions, a rotor 106 whose diameter is set so as to be equivalent to the inner diameter of the coil side 10s cannot be provided to the coil ends 10e, and therefore, the effective length of the motor is shortened, similarly.
On the other hand, as an example of the non-lap winding, there is Patent Literature 2.
In the case of the non-lap winding, since the coils do not lap over each other, the coil can be shaped into a simple shape. Therefore, the production cost is reduced. Further, the stator core can be provided over the whole length of the coil in the axial direction.
In the Patent Literature 2, in which the non-lap winding is employed, the pathways for water, air and the lighting electric wire are disposed at the outside of the stator core.